(a) Field of the Invention
The present invention relates to vacuum fluorescent displays (VFDs), and more particularly, to dynamic driving VFDs preventing erroneous activation of a phosphor layer by leakage electrons, and allowing a simpler wiring structure.
(b) Description of the Related Art
In recent years, VFDs, utilizing phosphor display elements to form a viewed alphanumeric or graphic image, have come into wide use as displays in electronic and electrical appliances.
A typical vacuum fluorescent display device comprises a transparent evacuated envelope containing a plurality of anodes arranged in a pattern of desired light emission, each anode being coated with a phosphor layer for emitting light when excited, a heated filament serving as a source of electrons, and mesh grids located between the filament and the anodes for determining which anodes can be excited by the electrons. When the anodes and the mesh grids are at a high voltage and the filament is at a lower voltage the electrons can excite the phosphor layer on the anodes to cause light emission from the anodes.
Referring to FIG. 1, a conventional VFD will be described. An evacuated envelope is sealed with a face glass 102, a base substrate 104 and side glasses 106. A wiring layer 108 is deposited on the base substrate 104 and covered with an insulating layer 110 except at through-holes. A conducting layer 114 (anode) is formed on the insulating layer 110 and provided with a positive potential through the through-holes. A phosphor layer 112 is deposited on the conducting layer 114.
A plurality of filamentary cathodes 116 is located in the envelope space with the anode 114, and is heated to thermionically emit electrons. A mesh grid 118 is located between the anode 114 and the cathodes 116 to accelerate the emitted electrons.
In the VFD shown in FIG. 1, or other similar triode vacuum tubes, the filament is heated, such as by an AC current, to a temperature at which it will emit electrons. The mesh grid biased at a positive voltage accelerates electrons emitted from the filament toward the anode, which is also biased higher than the filament bias. On the anode, the phosphor layer emits light in response to the bombardment of electrons emitted from the filament and accelerated by the mesh grid to the anode.
There are two types of driving types for the VFD, namely a dynamic driving type and a static driving type. In the static driving type VFDs, the anode acts to selectively actuate the indication pattern. When the anode is provided with a lower potential than that applied to the cathodes, the corresponding phosphor does not emits light. The mesh grid merely accelerates electrons.
In dynamic driving type VFDs, the mesh grid acts with respect to each of the indicating patterns to selectively actuate the indicating patterns. When the mesh grid is provided with a negative cut-off bias voltage (that is, a lower potential than that applied to the cathodes), the corresponding phosphor does not emits light. The mesh grid not only accelerates electrons, but also cuts off electrons.
The conventional fluorescent display tube of FIG. 1 has some drawbacks. First, the mesh grid is expensive, so it causes the manufacturing cost of VFDs to be high. Second, since only those electrons that pass through a positive mesh grid can reach the anode, some electrons that do not pass are absorbed by the mesh grid. That is, since only some of the emitted electrons impinge on the phosphor layer for lighting, the efficiency of electron function is not optimal and this results in a degradation of the brightness of the display.
Further, when the mesh grid is hit by the electrons, they may be heated by additional currents and suffer thermal deformation. Even if the spaces between grids are partially shifted, stains or spots will result on the display pattern.
In order to overcome the above drawbacks, another type of VFD has been proposed, in which electrically insulating ribs are formed on the substrate so as to surround respective phosphor layers. Further, grid electrodes are formed on the upper end faces of the ribs so that the grid electrodes are spaced from the upper surfaces of the phosphor layers in the direction perpendicular to the plane of the substrate.
FIG. 2 depicts a VFD having a similar structure to that shown in FIG. 1, except for an electrically insulating rib and grid electrode formed thereon. Each anode 114 and phosphor layer 112 is surrounded by an electrically insulating rib 118xe2x80x2 having a height of 100-150 xcexcm. A grid electrode 118xe2x80x3 is formed on the upper surface of the rib 118xe2x80x2 and has a height of 10-15 xcexcm.
The rib 118xe2x80x2 and grid electrode 118xe2x80x3 are generally formed by a thick-film printing process. The rib consists of a plurality of layers laminated by printing with an insulating paste. Each layer is printed in a thickness of 10-30 xcexcm and then dried. The printing and drying processes are repeated about three to fifteen times in order to complete each rib.
However, the conventional fluorescent display tube of FIG. 2 also has some drawbacks. First, it takes a long processing time since the insulating rib is laminated to a height of up to 100-150 xcexcm and the grid electrode is then formed on the rib.
Further, in dynamic driving, some of the electrons, which are emitted from the filament and accelerated by the grid electrode, may leak into the insulating layer 110. Specifically, the electrons leak between a rib whose grid electrode is applied by a positive potential and a rib whose grid electrode is applied by a negative cut-off bias voltage. The leakage electrons are then applied to the adjacent anode through the rib because the anode is biased with a positive voltage, and activate a phosphor layer (A in FIG. 2) below the electrode grid to which a negative cut-off bias voltage is applied. In this case, the phosphor layer which is not required to glow may glow due to the leakage electrons.
Further, since the grid electrode is formed on the upper surface of the rib, an intricate wiring structure is needed to apply a voltage to the grid electrode, resulting in complex manufacturing processes.
In view of the prior art described above, it is an object of the present invention to provide a dynamic driving vacuum fluorescent display capable of preventing erroneous activation of phosphor layers by leakage electrons.
It is another object of the present invention to provide a dynamic driving vacuum fluorescent display capable of allowing a simpler wiring structure.
To achieve these objects, as embodied and broadly described herein, the invention comprises
a substrate;
a plurality of anodes formed on the substrate, and phosphor layers formed on the respective anodes;
cathodes located above the phosphor layers to generate electrons which strike the phosphor layers; and
conductive ribs formed of an electrically conductive material on the substrate so as to surround at least a portion of a periphery of each of the phosphor layers and having a predetermined height.
Each of the conductive ribs not only accelerates the electrons when a positive voltage is applied, but also cuts off the electrons from the activation of the phosphor layers when a negative or zero cut-off voltage is applied.
The predetermined height of the conductive rib is more than 30 xcexcm, more preferably 60 xcexcm from a upper surface of the phosphor layer.
Both the foregoing general description and the following Detailed Description are exemplary and are intended to provide further explanation of the invention as claimed.